1. Field of the Invention
The invention is directed to a circuit for generating an AC voltage from a plurality of unregulated voltage sources having a temporally variable DC output voltage, such as photovoltaic installations or parts thereof, and a method for driving such a circuit. A “photovoltaic installation” should be understood here to mean exclusively the arrangement and electrical connection of a plurality of photovoltaic modules, as well as partial installations thereof. Such photovoltaic modules are connected together to form photovoltaic installations and have a design-dictated DC output voltage that is dependent on the level of incident sunlight and is thus temporally slowly variable, i.e., it varies over time. These changes in the DC output voltage have time constants on the order of minutes or longer.
2. Description of the Related Art
To feed the output current of photovoltaic installations into a power grid, whether public or local, the DC output voltage has to be converted, by means of an inverter circuit, into an AC voltage of constant frequency and voltage.
One exemplary circuit for generating such an AC voltage is described in DE 10 2008 034 955 A1, which discloses a power converter arrangement having a photovoltaic installation with a temporally varying output voltage, a level converter and an inverter. In the associated method, first and second capacitors of the level converter are charged independently of the input voltage with, in each case, half the value of the desired intermediate circuit voltage. What is disadvantageous about such known circuits is that, in such known circuits, the photovoltaic installation is loaded with a current that fluctuates for a short time in the frequency range of the driving of the level converter.
For such circuit arrangements there are a large number of application-dependent and partly conflicting requirements. By way of example, the individual photovoltaic modules are intended to be loaded as linearly as possible, i.e., their output current is intended to be as constant as possible over time within time intervals such as are typical of power semiconductor circuits, i.e., in the time range of less than one second, or in a frequency range above 1 Hz. Likewise, the input voltage at the inverter circuit is intended to be as constant as possible, and the transmission from the photovoltaic installation to the inverter circuit is intended to be effected with the highest possible voltage so that line losses are minimized.
Finally, the entire circuit is intended to operate with a high efficiency and in this case the number of components required is intended to be small. Their dimensioning, especially that of coils and capacitors, is likewise intended to be as small as possible. At the same time, the photovoltaic installation or the photovoltaic modules thereof should be operated in such a way that it operates at the point of maximum power output (MPP—maximum power point). Suitable regulation, so-called MPP tracking, is necessary for this purpose.